Acoustic resonators are used to process radio frequency (RF) electrical signals in various electronic applications. For example, acoustic resonators are often used as bandpass filters in cellular phones, global positioning system (GPS) devices, and imaging applications, to name but a few. Common forms of acoustic resonators include, for instance, bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) devices, thin film bulk acoustic resonator (FBAR) devices, double bulk acoustic resonator (DBAR) devices, and coupled resonator filter (CRF) devices.
FIGS. 1A through 1C are simplified diagrams of FBAR devices and are presented in order to illustrate certain principles of operation and construction that may apply to acoustic resonators more generally. In particular, FIG. 1A is a top view of an acoustic resonator 100, FIG. 1B is a cross-sectional view of acoustic resonator 100, taken along a line A-A′, and FIG. 1C is a cross-sectional view of an acoustic resonator 100′, which is a variation of acoustic resonator 100.
Referring to FIG. 1A, acoustic resonator 100 comprises a top electrode 125 having five (5) sides, with a connection side 101 configured to provide an electrical connection to electrical interconnect 102. Electrical interconnect 102 provides electrical signals to top electrode 125 to excite desired acoustic waves in a piezoelectric layer (not shown in FIG. 1A) of acoustic resonator 100. In alternative implementations, acoustic resonator 100 may have a different number of sides, i.e., less than 5 or greater than 5.
Referring to FIG. 1B, acoustic resonator 100 comprises an acoustic stack formed by a piezoelectric layer 120 disposed between a bottom electrode 115 and top electrode 125. The designations top electrode and bottom electrode are for convenience of explanation, and they do not represent any limitation with regard to the spatial arrangement, positioning, or orientation of acoustic resonator 100. The acoustic stack is disposed on a substrate 105 over an air cavity 110.
During typical operation, an electric field is applied between bottom electrode 115 and top electrode 125 of acoustic resonator 100. In response to this electrical field, the reciprocal or inverse piezoelectric effect causes acoustic resonator 100 to mechanically expand or contract depending on the polarization of the piezoelectric material. The presence of air cavity 110 prevents substrate 105 from absorbing mechanical energy of the expansion or contraction. As the electrical field varies over time, acoustic waves are generated in piezoelectric layer 120, and these acoustic waves propagate through acoustic resonator 100.
The frequency response of acoustic resonator 100 is determined by resonant frequencies of those propagating waves, which are influenced by the physical and electrical properties of acoustic stack and substrate 105, such as their dimensions, composition, impedance, mechanical stability, and so on. Accordingly, in an effort to improve the performance of acoustic resonators, researchers have explored various alternative ways of adjusting these and other properties. As one example, FIG. 1C shows a shows a variation of acoustic resonator 100 in which an acoustic stack is formed on a pedestal within an air cavity 110′. The pedestal structure mechanically and thermally isolates the acoustic stack from surrounding structures, which reduces the impact of mechanical stress on the membrane.
Referring to FIG. 1C, a substrate 105′ has an air cavity 110′ formed around a pedestal, and the acoustic stack comprises a bottom electrode 115′, a piezoelectric layer 120′, and a top electrode 125′. An interconnect layer 130′ extends along a bottom surface of air cavity 110′ to enable connection of the acoustic stack to external contacts, e.g., at connection side 101.
In general, electrical interconnects such as that provided by the extended bottom electrode 115′, or alternatively interconnections between different electrodes of a device or between different devices, may occupy an undesirably high amount of available device space, and they may also adversely affect performance of the acoustic resonators.
In view of these and other shortcomings of conventional interconnect technologies, there is a general need for electrical interconnection structures occupying a reduced amount of space and avoiding a negative impact on performance.